Variable stiffness coil for vasoocclusive devices

ABSTRACT

The variable stiffness vasoocclusive coil is given variable stiffness along the length of the coil by selectively heat treating certain segments of a primary or secondary coil. The primary coil can be selectively heat treated to form soft or deformable segments along the length of the coil, and can then be shaped into a secondary shape that is set by a heat treatment process. Distal regions of the coil can also be heat treated to make the distal ends of the coil softer, more deformable, or less traumatic.

RELATED APPLICATIONS

This is a continuation of Ser. No. 09/991,021, filed Nov. 15, 2001 nowU.S. Pat. No. 6,656,201 which is a divisional of Ser. No. 09/211,783,filed Dec. 15, 1998, now U.S. Pat. No. 6,383,204.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to implantable devices forinterventional therapeutic treatment or vascular surgery, and moreparticularly concerns a variable stiffness vasoocclusive coil thatexhibits variable stiffness along the length of the coil.

2. Description of Related Art

The art and science of interventional therapy and surgery hascontinually progressed towards treatment of internal defects anddiseases by use of ever smaller incisions or access through thevasculature or body openings in order to reduce the trauma to tissuesurrounding the treatment site. One important aspect of such treatmentsinvolves the use of catheters to place therapeutic devices at atreatment site by access through the vasculature. Examples of suchprocedures include transluminal angioplasty, placement of stents toreinforce the walls of a blood vessel or the like and the use ofvasoocclusive devices to treat defects in the vasculature. There is aconstant drive by those practicing in the art to develop new and morecapable systems for such applications. When coupled with developments inbiological treatment capabilities, there is an expanding need fortechnologies that enhance the performance of interventional therapeuticdevices and systems.

One specific field of interventional therapy that has been able toadvantageously use recent developments in technology is the treatment ofneurovascular defects. More specifically, as smaller and more capablestructures and materials have been developed, treatment of vasculardefects in the human brain which were previously untreatable orrepresented unacceptable risks via conventional surgery have becomeamenable to treatment. One type of non-surgical therapy that has becomeadvantageous for the treatment of defects in the neurovasculature hasbeen the placement by way of a catheter of vasoocclusive devices in adamaged portion of a vein or artery.

Vasoocclusion devices are therapeutic devices that are placed within thevasculature of the human body, typically via a catheter, either to blockthe flow of blood through a vessel making up that portion of thevasculature through the formation of an embolus or to form such anembolus within an aneurysm stemming from the vessel. The vasoocclusivedevices can take a variety of configurations, and are generally formedof one or more elements that are larger in the deployed configurationthan when they are within the delivery catheter prior to placement. Onewidely used vasoocclusive device is a helical wire coil having adeployed configuration which may be dimensioned to engage the walls ofthe vessels. One anatomically shaped vasoocclusive device that formsitself into a shape of an anatomical cavity such as an aneurysm and ismade of a pre-formed strand of flexible material that can be anickel-titanium alloy is known from U.S. Pat. No. 5,645,558, which isspecifically incorporated by reference herein. That vasoocclusive devicecomprises one or more vasoocclusive members wound to form a generallyspherical or ovoid shape in a relaxed state. The vasoocclusive memberscan be a helically wound coil or a co-woven braid formed of abiocompatible material, and the device is sized and shaped to fit withina vascular cavity or vesicle, such as for treatment of an aneurysm orfistula. The vasoocclusive member can be first helically wound orbraided in a generally linear fashion, and is then wound around anappropriately shaped mandrel or form, and heat treated to retain theshape after removal from the heating form. Radiopacity can be providedin the vasoocclusive members by weaving in synthetic or natural fibersfilled with powdered radiopaque material, such as powdered tantalum,powdered tungsten, powdered bismuth oxide or powdered barium sulfate,which can potentially be released during vascular surgery.

The delivery of such vasoocclusive devices can be accomplished by avariety of means, including via a catheter in which the device is pushedthrough the catheter by a pusher to deploy the device. The vasoocclusivedevices, which can have a primary shape of a coil of wire that is thenformed into a more complex secondary shape, can be produced in such away that they will pass through the lumen of a catheter in a linearshape and take on a complex shape as originally formed after beingdeployed into the area of interest, such as an aneurysm. A variety ofdetachment mechanisms to release the device from a pusher have beendeveloped and are known in the art.

For treatment of areas of the small diameter vasculature such as a smallartery or vein in the brain, for example, and for treatment of aneurysmsand the like, micro-coils formed of very small diameter wire are used inorder to restrict, reinforce, or to occlude such small diameter areas ofthe vasculature. A variety of materials have been suggested for use insuch micro-coils, including nickel-titanium alloys, copper, stainlesssteel, platinum, tungsten, various plastics or the like, each of whichoffers certain benefits in various applications. Nickel-titanium alloysare particularly advantageous for the fabrication of such micro coils,in that they can have super-elastic or shape memory properties, and thuscan be manufactured to easily fit into a linear portion of a catheter,but attain their originally formed, more complex shape when deployed.

One known technique for filling wide neck aneurysms involves breaking acoil or permanently deforming a coil within a vessel utilizing aballoon. However, substantial risks to a patient are involved in such aprocedure, and a coil which has soft or deformable segments may offerless risk to a patient. As a coil is inserted into the aneurysm, thecoil deforms and sets it shape, but over time a coil will typicallyassume its original shape, which is unlikely to correspond to the shapeof the vessel being filled. Filling of a variety of types of aneurysmsof various sizes and shapes may benefit by use of a variable stiffnesscoil that can deform more readily at certain predetermined sections. Assuch a variable stiffness coil is inserted into the aneurysm, the coilwill deform to conform to the shape and size of the vessel being filled,and will set its shape, but unlike a helical coil which over time takeson its original shape, a variable stiffness, deformable coil willpermanently deform in a random configuration, to thereby fill ananeurysm more evenly and completely over long periods of time.

A variable cross-section conical vasoocclusive coil is known that canachieve variations in stiffness of the coil by variation of the diameterin different regions of the coil or variations in the composition of thecoil. Methods are also known for construction of a stent with a varyingradial spring force, by heat treatments, by varying the stent framethickness, selectively machining stent ring frames, using differentalloys of the ring frames, and varying the Austenite finishtransformation temperature (Af) of a shape memory alloy such as Nitinol.A guide wire is also known that is formed from one or more heatactivated memory alloys, with intermediate portions that are selectivelyannealed to have variously curved shapes while the remainder of the wireremains straight when heated, and a stent is known that has U-shapedloop portions that are provided with greater flexibility by selectiveannealing to impart selective degrees of hardness to different portions.

It would be desirable to provide an vasoocclusive coil with primary andsecondary shapes with variable stiffness along the length of the coilthat can permanently deform in a random configuration that willpermanently deform in a random configuration in order to fill ananeurysm more evenly and completely over long periods of time. Thepresent invention meets these and other needs.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present invention provides for avariable stiffness vasoocclusive coil that exhibits variable stiffnessalong the length of the coil. Variable stiffness is accomplished byselectively heat treating certain segments of a primary or secondarycoil. The primary coil can be selectively heat treated to form soft ordeformable segments along the length of the coil, and can then be shapedinto a secondary shape that is set by a heat treatment process. Asecondary coil such as a three dimensional coil can be produced withvariable stiffness through a selective heating of localized segments ofthe coil. Distal regions of the coil can also be heat treated to makethe distal ends of the coil softer, more deformable, or less traumatic.Upon deployment, the coil will take on its pre-formed three dimensionalshape, and will deform in a random three-dimensional shape to conform tothe shape of the vessel or malformation into which the coil isintroduced. The variable stiffness coil is advantageously formed of ashape memory metal, and variable stiffness can be achieved through agingof desired segments of the shape memory metal coil to raise the parentphase or Austenite phase finish temperature, thus making the treatedsegments of shape memory metal softer and more flexible.

The invention accordingly provides for an occlusive device for use ininterventional therapy and vascular surgery adapted to be inserted intoa portion of a vasculature for occluding the portion of the vasculatureof a patient. The occlusive device comprises a variable stiffness coilformed from one or more flexible strands of a shape memory metal havinga primary coil configuration, the coil having a plurality of segmentsheat treated to cause the plurality of segments to have reducedstiffness. In one presently preferred embodiment, the variable stiffnesscoil has an expanded secondary coil configuration with a secondary threedimensional shape, such as a spherical or helical shape. In a preferredaspect, the flexible strand comprises a super-elastic material, whichcan be a shape memory metal such as a nickel titanium alloy. The shapememory nickel-titanium alloy is preferably heat treated such that thealloy is highly flexible at a temperature appropriate for introductioninto the vasculature via a catheter, and after placement, the devicewill take on a shape designed to optimize the therapeutic purposesdesired for the device.

The invention also provides for a method for making a variable stiffnessocclusive coil for use in interventional therapy and vascular surgeryadapted to be inserted into a portion of a vasculature for occluding theportion of the vasculature of a patient, comprising the steps ofproviding a coil formed from one or more flexible strands of a shapememory metal, the coil having a primary coil configuration and aninitial stiffness; and heat treating a plurality of segments of the coilto cause the plurality of segments to have reduced stiffness. In onepresently preferred embodiment, the step of providing a coil comprisesheating the coil in a desired three dimensional configuration to set thethree dimensional shape. In a preferred aspect of the method of theinvention, the shape memory metal has an Austenite phase finishtemperature, and the step of heating the coil comprises heating the coilat about 475° C. to 525° C. for about 1 to 20 minutes to set theAustenite phase finish temperature of the coil to about −5° C. to 10° C.The step of heat treating the coil can be accomplished by artificiallyaging a plurality of segments of the coil to raise the Austenite phasefinish temperature to about 35° C. to 50° C., such as by heating aplurality of segments of the coil to a temperature of about 400° C. fora period of about 5 seconds to 30 minutes.

These and other aspects and advantages of the invention will becomeapparent from the following detailed description and the accompanyingdrawings, which illustrate by way of example the features of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a primary helical vasoocclusive coil showing areas of heattreatment according to the invention.

FIG. 2 is a secondary helical vasoocclusive structure formed using theprimary helical coil of FIG. 1.

FIG. 3 is a secondary spherical vasoocclusive structure formed using theprimary helical coil of FIG. 1.

FIG. 4 is a graph illustrating the reduction in stiffness of a shapememory coil by heat treatment according to the principles of theinvention.

FIG. 5 is a schematic diagram of an apparatus for applying heat tosegments of a vasoocclusive coil to form a variable stiffnessvasoocclusive coil according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Modem techniques for filling wide neck aneurysms typically involvebreaking a coil or permanently deforming a coil within a vesselutilizing a balloon, with attendant substantial risks to a patient, anda coil which has soft or deformable segments may offer less risk to apatient. While modern vasoocclusive coils deform and set their shapewhen they are introduced into a vessel, over time such coils willtypically assume their original shape rather than to the shape of thevessel being filled. Filling of a variety of types of aneurysms ofvarious sizes and shapes may benefit by use of a variable stiffness coilthat can deform more readily at certain predetermined sections to fillan aneurysm more evenly and completely over long periods of time.

As is illustrated in the drawings, the invention is embodied in anocclusive device for use in interventional therapy and vascular surgeryadapted to be inserted into a portion of a vasculature for occluding aselected portion of the vasculature of a patient. In a presentlypreferred embodiment of the invention illustrated in FIG. 1, theocclusive device 10 is made from a strand of wire of approximately 0.001inch to approximately 0.006 inch in diameter and comprises a coil 12formed from one or more flexible strands of a super-elastic, shapememory metal such as nickel-titanium alloy, for example. While the abovestated range of diameters is presently known to be compatible with theinvention, larger or smaller diameters may be useful for particularapplications. The occlusive device typically has at least a primary coilconfiguration illustrated in FIG. 1, with a plurality of segments 14being heat treated to cause the plurality of segments to have reducedstiffness.

In one presently preferred embodiment shown in FIG. 2, the variablestiffness coil has an expanded secondary coil configuration with asecondary helical three dimensional shape 16, with localized heattreated segments 18, although the variable stiffness coil can also havean expanded secondary coil configuration with a secondary sphericalthree dimensional shape 20, with localized heat treated segments 22, asillustrated in FIG. 3. The shape memory metal is preferably heat treatedto be highly flexible at a temperature appropriate for introduction intothe vasculature via a catheter, and such that after placement, thedevice will take on a shape designed to optimize the therapeuticpurposes desired for the device.

The invention also provides for a method for making the variablestiffness occlusive coil. In a presently preferred embodiment, thevariable stiffness occlusive coil can be formed from a coil 12 of one ormore flexible strands of a superelastic shape memory metal. The coilpreferably has at least a primary coil configuration and an initialstiffness, as is illustrated in FIG. 4, representing the change instiffness of a heat treated segment of such a coil by application ofheat to the segment, such as by the apparatus shown in FIG. 5. Variablestiffness of the heat treated segment can be achieved through artificialaging of the shape memory metal, such as Nitinol. The shape memorybehavior of the shape memory metal can be modified by artificial agingof the material by heat treatment affecting the Austenitictransformation temperatures. When a shape memory alloy such as nickeltitanium alloy is deformed, and then heated to recover its originalparent or Austenite shape, the original shape corresponds to the shapeof the alloy in the relatively high temperature range of the parentphase. Once the Austenite phase finish temperature (Af) is reached, thenickel titanium alloy becomes stiffened. However, artificial aging ofthe nickel titanium alloy can raise the Af temperature, thus making thematerial act softer at higher temperatures. The coil is preferablyinitially heated in a desired three dimensional configuration to set thethree dimensional by heating the coil, such as in a salt pot, at about475° C. to 525° C. for about 1 to 20 minutes to set the Austenite phasefinish temperature of the coil to about −5° C. to 10° C. As isillustrated in FIG. 5, heat treating of a segment of the coil will causethe segment to have reduced stiffness, such as by artificially aging thesegment of the coil to raise the Austenite phase finish temperature toabout 35° C. to 50° C., by heating a plurality of segments of the coilto a temperature of about 400° C. for a period of about 5 seconds to 30minutes. This can be accomplished by placing the primary or secondaryshape coil in a heated air box 26 supplying hot air from a source ofheated air (not shown). The air box, typically made of brass, forexample, has a channel 30 in which the coil can be placed to expose thecoil 28 to a flow of hot air from a port 32 that is typically 0.020inches to 0.500 inches in diameter, conveyed to the port through aconduit 34 that extends through the air box. In this manner, localizedheating can be provided to desired portions of the coil, at controlledtemperatures for prescribed periods of time. Alternatively, heating ofsegments of the coil can be achieved by other means, such as by a laser,or by electrical heating, or other common types of heating elements.

It will be apparent from the foregoing that while particular forms ofthe invention have been illustrated and described, various modificationscan be made without departing from the spirit and scope of theinvention. Accordingly, it is not intended that the invention belimited, except as by the appended claims.

1. A method for making a variable stiffness occlusive coil for use ininterventional therapy and vascular surgery adapted to be inserted intoa portion of a vasculature for occluding the portion of the vasculatureof a patient, comprising the steps of: providing a coil formed from atleast one flexible strand of a flexible shape memory metal having anAustenite phase finish temperature, said coil having a primary coilconfiguration; and artificially aging a plurality of segments of saidcoil to cause said plurality of segments to have an Austenite phasefinish temperature that is greater than the Austenite phase finishtemperature of the remainder of the variable stiffness coil.
 2. Themethod of claim 1, wherein said step of providing a coil comprisingheating said coil in a desired three dimensional configuration to setsaid three dimensional configuration.
 3. The method of claim 1, whereinsaid step of artificially aging said plurality of segments of said coilraises the Austenite phase finish temperature of said plurality ofsegments of said coil to about 35° C. to 50° C.